The top bends (or knuckles) of the diagonal truss bars
extended above the top chords and were embedded into the
concrete slab to make the whole system composite.

The floor system spanned between the external walls and
the core. At the external walls, truss top chords were bolted
to the seats welded to the spandrels. At the central core,
trusses were bolted to seats welded to a girder supported
by the core columns.

Viscoelastic dampers were installed between bottom chords
and spandrels to reduce the sway and vibration of the buildings
by winds.

The floors supported the gravity loads and provided lateral
stability to the external walls.

Hat truss system

Located between the 107th and 110th floors.

The system comprised a set of steel braced trusses, designed
to support a tall antenna on top of each tower (only WTC
1 had one installed).

The system provided additional connections among the core
columns and between the core and external columns, providing
additional means for load redistribution.

Fire Protection Systems

The fire protection system installed in the WTC towers at the
time of terrorist attack was summarised as follows:

Fire Protection System

Remarks

Fire compartmentation

In the core area, the stairwells and
elevator shafts were enclosed by 2 hour fire-rated walls
made of gypsum wallboard on steel studs.

In the open-plan tenant area, 1 hour fire-rated floor-to-slab
partitions were used to separate the tenant spaces from each
other and from the common core area.

Firestopping materials were used to fill gaps in walls
and floors to prevent the spread of smoke and flames.

The external wall was connected with the floors without
gaps.

Fire protection to steelwork

Most of the core columns were protected
by gypsum wallboard.

Slab trusses, perimeter columns, spandrels and some faces
of core columns were coated with three different sprayed
fire protection materials.

Sprinkler system

Automatic sprinklers had been installed
on all floors, capable of controlling local fires totalling
an aggregate floor area of up to 418 m2 (4,500 ft2).

Fire alarm system

Multifunction fire alarm system was installed,
capable of alerting staff at the Fire Command Station within
the building and providing voice and strobe alerts throughout.

Dry riser system

Standpipes were installed in the stairwells
one each floor, gravity fed from 75,708 L (20,000 gal) of
stored water and by three large water pumps.

Plane Crash and Damage

On 11 September 2001, two passenger planes were hijacked by
terrorists and crashed into the World Trade Center Towers in
New York.

When the two aircrafts crashed into the towers, fireballs erupted
and jet fuel spread across the impact floors and down internal
shafts, igniting multiple floor fires immediately. The resulting
fires spread throughout the upper floors above the impact floors
of the two towers. The twin towers collapsed shortly afterwards.

Based on the available documents, photographs, video footages,
steel data collection and computer modelling, National Institute
of Standards and Technology (NIST) of USA produced the reconstruction
of the events that accompanied and followed the plane clashes.

The timeline for the collapse of WTC 1 according to FEMA 403
and NIST NCSTAR 1 (Draft) is summarised as follows:

Time

Damage and Fire

08:46:30 EDT
(GMT - 4 hours)

A hijacked Boeing 767-200ER airplane
crashed into the north face of WTC 1 between the 94th and
98th floors, directly causing

Jet fuel erupted into fireballs and ejected from
the 79th, 81st and 82nd floors

Fireballs burned for 10 sec, extending almost 61
m (200 ft) out from the north, east and south faces

Vigorous fires on the east side of the 80th to
83rd
floors

Within 18 min of the crash, the east perimeter columns
between the 79th to 83rd floors had bowed
inward due to floors sagging

09:30
~ 09:58

Fires continued to burn in the east
half of the tower

09:58:59

The top of the tower tilted to the east and south,
and WTC 2 collapse

Analysis

The impact of the plane crashes directly caused significant
structural damages to both WTC towers. The towers resisted this
level of damage and did not collapse immediately showing that
the redundancy of the tube-frame structures enabled the redistribution
of the loads from the damaged zones to the remaining structures.

However, it was generally believed that the impact also extensively
striped off the fire protection materials from structural steel
at least in the crashed zones. The multiple floors fires ignited
by the jet fuel finally weakened the remaining structures and
the towers collapsed.

It is worth noting that in the tube-frame system of the WTC
towers, the lateral resistance or stability of the perimeter
columns were provided by the composite floor truss system. This
lateral restraint is reduced as the floor trusses weaken and
sag in the heat. In a multiple floors fire, it was expected the
effective length of laterally unrestrained perimeter columns
would increase at least twice or triple. In addition to the direct
thermal effects, the compressive resistance of these columns
eventually reduced until a point that they could not sustain
the applied load and buckling occurred.

The video footages showed that in WTC 1, the south perimeter
columns of the fire floors had buckled before the top section
of the tower tilted to the south and collapsed. On the other
hand, in WTC 2, the east perimeter columns of the fire floors
had also buckled before the top section of the tower tilted to
the east and south and collapsed.

[NIST NCSTAR 1 (Draft)] National Institute of Standards and
Technology. (2005). Federal Building and Fire Safety Investigation
of the World Trade Center Disaster: Final Reports of the National
Construction Safety Team on the Collapses of the World Trade
Center Towers (Draft), U.S. Government Printing Office, Washington.